Positron emission tomography (PET) is becoming increasingly widely established alongside magnetic resonance tomography (MR) in medical diagnostics. While MR is an imaging method for representing structures and slices inside the body, PET allows in vivo visualization and quantification of metabolic activities.
PET uses the special properties of positron emitters and positron annihilation in order to quantitatively determine the function of organs or cell regions. With this technique, appropriate radiopharmaceuticals marked with radionuclides are administered to the patient prior to the examination. As they decay, the radionuclides emit positrons which after a short distance interact with an electron, causing what is termed annihilation to occur. This results in two gamma quanta which fly apart in opposite directions (offset by 180°). The gamma quanta are detected by two opposing PET detector modules within a specific time window (coincidence measurement), as a result of which the annihilation site is localized to a position on the line connecting said two detector modules.
In the case of PET, the detector module must generally cover a greater part of the gantry arc length for the purpose of detection. It is subdivided into detector elements having a side length of a few millimeters. On detecting a gamma quantum, each detector element generates an event record that specifies the time and the detection location, i.e. the corresponding detector element. This information is passed to a fast logic unit and compared. If two events coincide within a maximum time interval, it is assumed that a gamma decay process is taking place on the connecting line between the two associated detector elements. The PET image is reconstructed using a tomography algorithm, i.e. so-called back-projection.
It is known to combine PET with other tomographic methods, in particular computed tomography. In combined PET/CT scanners it is possible, for example, to compensate for the lack of spatial resolution of PET systems. At the same time CT provides a visualization of the patient's anatomy, so that when the CT and PET data are mutually superimposed it is possible to establish precisely where in the body the PET activity occurred. In combined PET/CT scanners a PET scanner and CT scanner are typically arranged one behind the other such that the patient can be transferred seamlessly from one device to the other during an examination. The two measurements can then be performed in immediate succession.
It is advantageous to combine a PET scanner with an MR scanner because MR offers a higher soft tissue contrast than CT. Combined MR/PET systems are already known in which the PET detectors are arranged within an opening defined by the MR magnet together with the gradient system and excitation coil. In this arrangement they are positioned next to the excitation coil so that the target examination volumes of the MR and PET systems do not coincide but are offset in the z direction. Analogously to the PET/CT system it is consequently not possible here to measure PET and MR data simultaneously.
It is particularly preferred in this case for the PET scanner to be arranged inside the MR scanner and for the two examination volumes to be mutually superimposed. It will then be possible to acquire both morphological MR data and PET data during a single measurement pass. Apart from the time-saving effect, both image data sets can be presented in a simple manner, mutually superimposed so that a diagnosis will be made easier for the physician.
In order to integrate the PET scanner and MR scanner it is necessary to arrange the PET detectors inside the MR device so that the imaging volumes will be positioned isocentrically. For example, the PET detectors can be arranged on a support structure (support tube, gantry) located inside the MR device. These can consist of, for example, 60 detectors disposed in an annular arrangement on the support tube. A connected cooling means and electrical supply lines are required for each of the detectors, which can also be combined into detector blocks. These must likewise be arranged in the MR scanner. A number of signal processing units are additionally required that are likewise arranged in the MR scanner. Said units are connected to the detectors via the electrical supply lines and serve for signal processing.
In the case of a combination of MR and PET in a combined system, however, the gamma quanta are attenuated by anything situated between the site of origin of the respective gamma quanta and the PET detector. The attenuation must be taken into account in the reconstruction of PET images in order to prevent image artifacts. Situated between the site of origin of the gamma quantum in the patient's body and the acting PET detector are tissue within the patient's body as well as air, generally, and a part of the MR/PET system itself, for example a cover of the patient bore or a patient positioning table. The attenuation values of the components or accessory parts requiring to be taken into account are compiled into attenuation maps (μ maps). An attenuation map contains attenuation values for each volume element (voxel) of the volume under examination. Thus, for example, an attenuation map can be produced for the patient positioning table. The same applies to, for instance, local coils attached to the patient for MR examinations. In order to produce the attenuation map it is necessary to ascertain and combine the attenuation values. They can be ascertained by means of, for example, a CT recording or PET transmission measurement of the respective component. Attenuation maps of said kind can be measured on a once-only basis, since the attenuation values do not change over the life of the respective component. For the attenuation correction, great differences in the attenuation between the different tissues, especially air, soft parts and bone, are of primary importance.
It is known in the case of PET/CT systems to calculate an attenuation map from CT recordings using the X-ray absorption coefficients and use it to correct the attenuation of PET data. This can also be employed in measuring attenuation values of the components. It is not possible in the case of PET systems to ascertain the attenuation map directly from the actual measurement data. It must be measured in test measurements using homogeneous PET phantoms so that the intensity of the resulting gamma quanta will be known.
Methods are known by which attenuation values of the patient's body can be determined from anatomical MR images and can be added to the attenuation map. In this case special MR sequences are used by means of which bones, for example, can be identified. With the aid of the MR images it is then possible, based on knowledge of the position of the bones in the beam path of the gamma quanta, to appropriate attenuation values to the attenuation map.
However, the imaging volume of the MR scanner is generally not large enough for imaging the entire patient and thus providing attenuation values for the entire patient. Although it is possible in principle, by taking a plurality of measurements for example, to image the torso and the arms by means of MR and thereby determine the attenuation values, this requires an increased amount of time. It is also possible to increase the size of the imaging volume of the MR scanner by structural measures to such an extent that the entire anatomy of a patient can be recorded. Scanners of this type are extremely expensive, however. It is desirable to be able to determine attenuation values also for MR scanners having smaller imaging volumes.